In an optical disc drive that is capable of optically recording or reading data using a semiconductor laser as a light source, a CD (compact disc) in which the semiconductor laser has its center wavelength of 780 nm and a DVD (digital versatile disc) in which the semiconductor laser has its center wavelength of 650 nm have been made practical. Furthermore, generally, compatibility between both kinds of the optical discs is attained so that a single optical disc drive can handle both optical discs.
In such compatible optical disc drives, there are mounted two types of semiconductor lasers, a laser that generates an output beam at a wavelength of 780 nm and an output beam at 650 nm, on an optical head by which data are recorded on or read from an optical disc.
A conventional optical head has a tracking correction function that correctly tracks a data track on the optical disc; furthermore, generally, in order to perform such tracking correction, the optical head is generally provided with a diffraction grating that disperses into three light beams a light beam from the semiconductor laser. For example, diffraction grating specifications have been designed to make implementable a tracking error detection method called three-beam method, well known in the CD, and a tracking error detection method called differential push-pull method, well known in the DVD. Thus, a dedicated diffraction grating is needed for each wavelength of the semiconductor lasers.
However, recent technological advances in semiconductor laser have made it practical to provide a semiconductor laser that enables laser beams of two different wavelengths to generate from a single semiconductor laser package, or a single semiconductor laser element. A problem has been that since these two different-wavelength laser beams travel along the same optical path, desired optical performance for the laser beam of each of the two wavelengths can not be achieved when a diffraction grating designed to satisfy optical performance in a conventional single wavelength is used.
There is disclosed an improvement for overcoming this problem: a diffractive optical element and an optical head that is provided thereon with a diffractive optical element in which only a first diffraction grating works for a first wavelength (e.g., 780 nm) and only a second grating for a second wavelength (e.g., 650 nm), by using a diffractive optical element integrally having two diffraction gratings (for example, refer to Patent Document 1).
In addition there is also disclosed a diffractive optical element and an optical head provided with the element in which a width ratio between diffraction grating ungrooved portions and grooved portions is optimized in order to adjust a dispersion rate of a laser beam on the respective gratings (for example, refer to Patent Document 2).
Patent Document 1
Japanese Unexamined Patent Publication 2002-190133 (pages 4 and 5, FIG. 1)
Patent Document 2
Japanese Unexamined Patent Publication 2002-311219 (pages 4 and 5, FIG. 2)